The cells of the adenohypophysis make up the intermediate level of the endocrine axes. The adenohypophysis secretes protein hormones that are referred to as tropic hormones—adrenocorticotropic hormone (ACTH, also called corticotropin), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), growth hormone (GH), and prolactin (PRL) (see
Table 41.1). With a few exceptions, tropic hormones bind to their cognate receptors on peripheral endocrine glands. Because of this arrangement, pituitary tropic hormones generally do not directly regulate physiological responses (see
Chapter 38).
The endocrine axes have the following important features: 1.
The activity of a specific axis is normally maintained at a set point, which varies from individual to individual, usually within a normal range. The set point is determined by the integration of hypothalamic stimulation and peripheral hormone negative feedback. Importantly, negative feedback generally is not exerted by the physiological responses regulated by a specific endocrine axis but by the peripheral hormone itself acting on the pituitary and hypothalamus (see
Fig. 41.6 ). Thus if the level of a peripheral hormone drops, secretion of hypothalamic releasing hormones and pituitary tropic hormones will increase. As the level of peripheral hormone rises, the hypothalamus and pituitary will decrease secretion because of negative feedback. Although certain nonendocrine physiological parameters (e.g., acute hypoglycemia) can regulate some endocrine axes, the axes function semiautonomously with respect to the physiological changes they produce. This configuration means a peripheral hormone (e.g., thyroid hormone) can regulate multiple organ systems without these organ systems exerting competing negative-feedback regulation of the hormone. Clinically this partial autonomy means that multiple aspects of a patient’s physiology are at the mercy of whatever derangements might exist within a specific axis.
2.
Hypothalamic hypophysiotropic neurons are often secreted in a pulsatile manner and are entrained to daily and seasonal rhythms through CNS input. Additionally, hypothalamic nuclei receive a variety of neuronal inputs from higher and lower levels of the brain. These can be • Fig. 41.5 Synthesis, processing, and transport of preprovasopressin. Human ADH (also called arginine vasopressin [AVP]) is synthesized in the hypothalamic magnocellular cell bodies and packaged into neurosecretory granules. During intraaxonal transport of the granules down the infundibular process to the pars nervosa, provasopressin is proteolytically cleaved into the active hormone (AVP = ADH), neurophysin (NP), and a C-terminal glycoprotein (GP). NP arranges into tetramers that bind five AVP molecules. All three fragments are secreted from axonal termini in the pars nervosa (posterior pituitary) and enter the systemic blood. Only AVP (ADH) is biologically active. (Modified from Larsen PR et al [eds]. Williams Textbook of Endocrinology. 10th ed. Philadelphia: Saunders; 2003.)